Add serialization of TypedExpr to proto

[johnynek]

part of #54

This adds conversion of TypedExpr to proto. Along the way, it also makes TypeExpr covariant.

[codecov-io]

Codecov Report

Merging #266 into master will increase coverage by 0.1%.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##           master     #266     +/-   ##
=========================================
+ Coverage   85.93%   86.04%   +0.1%     
=========================================
  Files          55       55             
  Lines        4082     4184    +102     
  Branches      171      179      +8     
=========================================
+ Hits         3508     3600     +92     
- Misses        574      584     +10

Impacted Files	Coverage Δ
...re/src/main/scala/org/bykn/bosatsu/TypedExpr.scala	71.01% <60%> (+0.86%)	⬆️
...main/scala/org/bykn/bosatsu/TypedExprToProto.scala	87.52% <91.37%> (+1.06%)	⬆️
...e/src/main/scala/org/bykn/bosatsu/rankn/Type.scala	89.47% <0%> (-0.88%)	⬇️

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